Context :

Aims : We report on a detection of an early rising phase of optical afterglow ( OA ) of a long GRB 090726 .
We resolve a complicated profile of the optical light curve .
We also investigate the relation of the optical and X-ray emission of this event .

Methods : We make use of the optical photometry of this OA obtained by the 0.5 m telescope of AI AS CR , supplemented by the data obtained by other observers , and the X-ray Swift /XRT data .

Results : The optical emission peaked at \sim 17.5 mag ( R ) at t - T _ { 0 } \approx~ { } 500 s. We find a complex profile of the light curve during the early phase of this OA : an approximately power-law rise , a rapid transition to a plateau , a weak flare superimposed on the center of this plateau , and a slowly steepening early decline followed by a power-law decay .
We discuss several possibilities to explain the short flare on the flat top of the optical light curve at t - T _ { 0 } \approx 500 s ; activity of the central engine is favored although reverse shock can not be ruled out .
We show that power-law outflow with \Theta _ { obs } / \Theta _ { c } > 2.5 is the best case for OA of GRB 090726 .
The initial Lorentz factor is \Gamma _ { 0 } \approx 230 - 530 in case of propagation of the blast wave in a homogeneous medium , while propagation of this wave in a wind environment gives \Gamma _ { 0 } \approx 80 - 300 .
The value of \Gamma _ { 0 } in GRB 090726 thus falls into the lower half of the range observed in GRBs and it may even lie on the lower end .
We also show that both the optical and X-ray emission decayed simultaneously and that the spectral profile from X-ray to the optical band did not vary .
This is true for both the time period before and after the break in the X-ray light curve .
This break can be regarded as achromatic .
The available data show that neither the dust nor the gaseous component of the circumburst medium underwent any evolution during the decay of this OA , that is after t - T _ { 0 } < 3000 s. We also show that this OA belongs to the least luminous ones in the phase of its power-law decay corresponding to that observed for the ensemble of OAs of long GRBs .

Conclusions :